Group delay is a measure of the rate of change of phase shift through a device as a function of frequency and is therefore a very useful measurement for characterizing the performance of RF and microwave devices. Since a nonlinear change in phase shift with frequency can produce distortion in a signal, group delay provides a good measure of the distortion produced by a device. Thus it is common to measure and specify the group delay of filters, amplifiers, mixers, cables and other transmission devices.
In the past, group delay has been measured at a single frequency by changing slightly the frequency of the signal passing through the test device and noting the change in phase. One can then make a calculation of the change in phase over the change in frequency which gives the average group delay of the range over which the frequency was changed. Single frequency measurements, however, are inconvenient when a broad band device is being tested since much time and effort is required to make measurements over any appreciable band of frequencies.
Prior art methods are also available for measuring group delay on a swept frequency basis. Such a method is described, for example, in Hewlett-Packard Application Note 77-4 entitled "Swept Frequency Group Delay Measurements," 1968. In the method described in the Application Note, a swept frequency signal generator, or sweeper, supplies the device under test with a signal whose frequency changes linearly with time. This signal is modulated at some convenient lower frequency so that an amplitude modulated signal passes through the device under test. The output signal from the device under test is passed through a detector to recover the modulation signal; and a ratio voltmeter, such as the Hewlett-Packard Model 8405 Vector Voltmeter, is connected to measure both the original modulation signal and the detected modulation signal. The voltmeter indicates the phase difference between the two signals; and, given the modulation frequency, this phase information can be transferred into group delay.
This method has, however, several inherent disadvantages. The modulation frequency is a measure of the smallest band width within which fluctuations in delay can still be measured, and the period of the modulation frequency is equal to the largest group delay measurable. Thus, ideally, the smaller the modulation frequency the larger and more accurate a measurement can be made. However, with smaller modulation frequencies the resultant phase shift for any given delay will be smaller and harder to measure. Thus the resolution will be decreased as the frequency decreases. On the other hand, increasing the frequency to increase the resolution decreases accuracy and also runs into the problem that the modulation frequency may be greater than the passband of the device under test. Furthermore, this method requires the measurement of the phase between two sinusoidal signals: the modulation signal and the detected modulation signal. Any distortion introduced in the modulation signal by the device under test can lead to errors in the measurement of the phase and thus lead to errors in the measurement of group delay.